00:00:21.130 you 00:00:41.58000:00:41.590 welcome back the objectives of this 00:00:44.17000:00:44.180 particular lecture are to describe 00:00:47.23000:00:47.240 salient features of some of the 00:00:48.67000:00:48.680 important types of evaporators discuss 00:00:53.23000:00:53.240 thermal design aspects of refrigerant 00:00:54.97000:00:54.980 evaporators present correlations for 00:00:58.27000:00:58.280 estimating heat transfer coefficients 00:01:00.04000:01:00.050 and discuss Wilson's plots so at the end 00:01:05.17000:01:05.180 of the lecture you should be able to 00:01:07.62000:01:07.630 describe important features of some of 00:01:09.91000:01:09.920 the important types of evaporators 00:01:12.21000:01:12.220 explain the complexities to be 00:01:14.23000:01:14.240 considered in the design of refrigerant 00:01:15.85000:01:15.860 evaporators estimate heat transfer 00:01:19.06000:01:19.070 coefficients using the correlations 00:01:20.62000:01:20.630 presented and finally use the concept of 00:01:23.77000:01:23.780 wilson's plot and estimate various heat 00:01:25.69000:01:25.700 transfer coefficients so let us look at 00:01:30.34000:01:30.350 a very important type of evaporator 00:01:32.65000:01:32.660 called a shell and tube type evaporator 00:01:35.79000:01:35.800 the as the name implies this kind of an 00:01:38.32000:01:38.330 evaporator consists of a shell and a 00:01:39.94000:01:39.950 large number of straight tubes arranged 00:01:42.07000:01:42.080 parallel to each other in dry expansion 00:01:45.73000:01:45.740 type the refrigerant flows through the 00:01:47.62000:01:47.630 tubes while in flooded time or 00:01:49.38000:01:49.390 refrigerant flows through the shell so 00:01:51.49000:01:51.500 let me show the picture of a dry 00:01:53.41000:01:53.420 expansion type shell and tube type 00:01:54.70000:01:54.710 evaporator so this particular picture 00:01:57.52000:01:57.530 shows that a shell and tube type 00:01:58.96000:01:58.970 evaporator where the refrigerant flows 00:02:01.54000:02:01.550 through the tubes you can see that the 00:02:02.86000:02:02.870 refrigerant flows through the tubes 00:02:05.41000:02:05.420 while the water flows through the shell 00:02:07.44000:02:07.450 okay so this is a two pass you can call 00:02:11.05000:02:11.060 this as a to pass as far as the 00:02:12.34000:02:12.350 refrigerant is concerned because the 00:02:13.66000:02:13.670 refrigerant flows through tubes like 00:02:15.34000:02:15.350 this then takes u-turn and again it 00:02:17.77000:02:17.780 flows in this direction and finally 00:02:19.30000:02:19.310 leaves the evaporator here okay in any 00:02:24.37000:02:24.380 shell and tube type of evaporators 00:02:26.02000:02:26.030 baffles are provided in the shell side 00:02:28.72000:02:28.730 you can see that these are the baffles 00:02:30.16000:02:30.170 shown okay the purpose of the baffles is 00:02:33.13000:02:33.140 to avoid stagnant zones and also to 00:02:35.77000:02:35.780 improve turbulence so that you can get 00:02:37.39000:02:37.400 good heat transfer coefficient on the 00:02:39.28000:02:39.290 shell side for example you can see that 00:02:40.84000:02:40.850 because of the presence of the baffle 00:02:42.43000:02:42.440 water is forced to flow like this okay 00:02:45.75900:02:45.769 so short circuiting of water is 00:02:47.41000:02:47.420 prevented thereby you can get good heat 00:02:49.42000:02:49.430 transfer coefficient and the stagnant 00:02:51.22000:02:51.230 regions can be 00:02:52.66000:02:52.670 of course a baffle so increase pressure 00:02:54.97000:02:54.980 drop through the shell side so the 00:02:57.07000:02:57.080 baffles have got to be optimized so this 00:03:00.34000:03:00.350 is the direct side expansion type shell 00:03:03.22000:03:03.230 and tube type evaporator as I said since 00:03:05.14000:03:05.150 the refrigerant flows through the tubes 00:03:07.23000:03:07.240 at the end of the tube you can have 00:03:09.55000:03:09.560 super heating okay that means what 00:03:11.02000:03:11.030 refrigerant goes out can be a 00:03:12.82000:03:12.830 superheated vapor okay now let us look 00:03:16.99000:03:17.000 at the other features the shell 00:03:22.80900:03:22.819 diameters normally range from 150 mm to 00:03:25.27000:03:25.280 1.5 meters the number of cubes may be 00:03:28.84000:03:28.850 less than 50 to several thousands and 00:03:30.97000:03:30.980 the length of the heat exchanger may be 00:03:33.22000:03:33.230 between 1.5 meter to 6 meters normally 00:03:38.56000:03:38.570 steel tubes are used with ammonia while 00:03:40.27000:03:40.280 copper tubes are used with freon and 00:03:42.13000:03:42.140 when we use freons the heat transfer 00:03:43.69000:03:43.700 coefficient obtained on the refrigerant 00:03:45.67000:03:45.680 side is typically small so we normally 00:03:47.94900:03:47.959 use what is known as integrally fin 00:03:50.05000:03:50.060 tubes on the refrigerant side whereas in 00:03:52.87000:03:52.880 case of ammonia since the heat transfer 00:03:54.49000:03:54.500 coefficient is large fins are not 00:03:56.28900:03:56.299 required their dry expansion type 00:04:02.17000:04:02.180 evaporators uses fins inside the tube 00:04:04.66000:04:04.670 that means on the refrigerant side while 00:04:06.52000:04:06.530 flooded type uses fins outside the tubes 00:04:08.80000:04:08.810 because in the flooded type refrigerant 00:04:10.60000:04:10.610 flow from the outside of the tubes dry 00:04:13.87000:04:13.880 expand expansion type requires less 00:04:15.88000:04:15.890 refrigerant and has positive lubricating 00:04:18.43000:04:18.440 oil returned that means the lubricating 00:04:19.90000:04:19.910 oil returned to the compressor is 00:04:21.13000:04:21.140 ensured in dry expansion type and the 00:04:23.74000:04:23.750 amount of refrigerant charge required 00:04:25.30000:04:25.310 inside the system is also less and these 00:04:29.95000:04:29.960 dry expansion type shell and tube type 00:04:31.63000:04:31.640 evaporators are normally used for small 00:04:33.88000:04:33.890 and medium capacity refrigeration plants 00:04:35.71000:04:35.720 that means the capacity ranges from 00:04:37.81000:04:37.820 about 2 tons to 350 tons whereas the 00:04:42.31000:04:42.320 flooded type evaporators are available 00:04:44.08000:04:44.090 in larger capacities ranging from 10 00:04:46.48000:04:46.490 tons to thousands of tons okay in a very 00:04:49.65900:04:49.669 large plants flooded type evaporators 00:04:51.49000:04:51.500 are used so now let us look at flooded 00:04:56.23000:04:56.240 type shell and tube evaporator as I have 00:05:00.90900:05:00.919 already mentioned the liquid that means 00:05:02.40900:05:02.419 the external fluid which is usually 00:05:05.38000:05:05.390 the brine or water which is to be 00:05:07.12000:05:07.130 chilled flows through the tubes and the 00:05:10.63000:05:10.640 refrigerant is fed through a float valve 00:05:12.31000:05:12.320 which maintains a constant level of 00:05:13.99000:05:14.000 liquid refrigerant in the shell both 00:05:17.62000:05:17.630 single and multi pass arrangements are 00:05:19.15000:05:19.160 possible and these are also available in 00:05:22.33000:05:22.340 vertical configuration since the shell 00:05:26.05000:05:26.060 is not completely filled with liquid 00:05:27.73000:05:27.740 refrigerant super heating is also 00:05:28.87000:05:28.880 possible so this picture shows the 00:05:32.11000:05:32.120 flooded type shell and tube evaporator 00:05:35.08000:05:35.090 as I have already told in this case 00:05:37.53000:05:37.540 refrigerant is on the shell side okay 00:05:40.30000:05:40.310 where are the water flows through the 00:05:41.86000:05:41.870 tube side again this is a two pass 00:05:43.72000:05:43.730 arrangement as far as water is concerned 00:05:45.10000:05:45.110 because water flows through the tubes 00:05:46.84000:05:46.850 like this takes a u-turn and again flows 00:05:49.54000:05:49.550 back in this direction okay so that 00:05:51.97000:05:51.980 means the water crosses the refrigerant 00:05:53.89000:05:53.900 path twice so there you call this as two 00:05:56.17000:05:56.180 paths arrangement okay 00:05:58.00000:05:58.010 you can see that the this is the liquid 00:06:02.23000:06:02.240 refrigerant level okay liquid 00:06:05.11000:06:05.120 refrigerant level so the this is 00:06:09.19000:06:09.200 maintained by a float one and the entire 00:06:11.50000:06:11.510 shell is not filled with liquid okay you 00:06:13.36000:06:13.370 can see that there is an empty space in 00:06:15.19000:06:15.200 which refrigerant vapour is there but 00:06:16.84000:06:16.850 not the liquid okay this is required to 00:06:19.21000:06:19.220 prevent the exit of liquid refrigerant 00:06:21.97000:06:21.980 liquid droplets along with the vapour 00:06:24.16000:06:24.170 okay and since you have some space above 00:06:27.43000:06:27.440 the liquid level super heating is also 00:06:29.14000:06:29.150 possible in this kind of arrangement 00:06:31.71000:06:31.720 okay 00:06:37.75000:06:37.760 now let us look at shell end coil type 00:06:40.94000:06:40.950 evaporator these are of smaller capacity 00:06:44.36000:06:44.370 than the shell and tube type these are 00:06:48.08000:06:48.090 made of one or more spiral shape bare 00:06:50.09000:06:50.100 tube coils enclosed in a welded shell so 00:06:52.73000:06:52.740 this is almost similar to shell and coil 00:06:54.44000:06:54.450 type condenser which was discussed in an 00:06:56.57000:06:56.580 earlier lecture insulin call type 00:07:00.68000:07:00.690 evaporator refrigerant can flow through 00:07:02.36000:07:02.370 the coil in direct expansion type or 00:07:04.79000:07:04.800 through the shell insulated type in 00:07:09.29000:07:09.300 direct expansion type the large amount 00:07:11.18000:07:11.190 of water in shell provides what is known 00:07:13.37000:07:13.380 as a holdup capacity that means a large 00:07:16.04000:07:16.050 thermal storage capacity is provided by 00:07:18.41000:07:18.420 the amount of water in the shell and 00:07:19.79000:07:19.800 this square is known as holdup capacity 00:07:22.45000:07:22.460 and this type is good for small but 00:07:26.12000:07:26.130 highly infrequent peak loads now let us 00:07:30.98000:07:30.990 look at double pipe type evaporator as 00:07:32.72000:07:32.730 the name implies this is almost similar 00:07:34.55000:07:34.560 to in construction to the double pipe 00:07:36.62000:07:36.630 type condenser they again this consists 00:07:41.06000:07:41.070 of two concentric tubes the refrigerant 00:07:43.10000:07:43.110 flows through the annular passes while 00:07:44.81000:07:44.820 the liquid being chilled flows to the 00:07:46.76000:07:46.770 inner tube in parallel flow or counter 00:07:48.86000:07:48.870 flow these may be used in flooded as 00:07:52.67000:07:52.680 well as direct expansion mode this type 00:07:56.51000:07:56.520 of evaporator requires more space than 00:07:58.79000:07:58.800 other designs shorter tubes and counter 00:08:02.48000:08:02.490 flow gives good heat transfer 00:08:03.44000:08:03.450 coefficient and outer tubes need 00:08:07.25000:08:07.260 insulation to minimize heat leaks 00:08:08.90000:08:08.910 insulation is required because there are 00:08:10.85000:08:10.860 refrigerant flows through the annular 00:08:12.29000:08:12.300 space okay and refrigerant temperature 00:08:14.21000:08:14.220 is much lower than the ambient 00:08:15.65000:08:15.660 temperature so to reduce heat leaks you 00:08:18.38000:08:18.390 have to insulate the outer tubes as far 00:08:20.99000:08:21.000 as the space occupied is concerned the 00:08:23.75000:08:23.760 shell and tube type evaporators occupy 00:08:26.12000:08:26.130 much less space compared to other types 00:08:28.10000:08:28.110 of evaporators and the volume is also 00:08:30.89000:08:30.900 quite small okay that is the reason why 00:08:32.69000:08:32.700 the shell and tube type of evaporators 00:08:34.25000:08:34.260 are very commonly used in large capacity 00:08:36.82900:08:36.839 systems okay whereas the tube in tube 00:08:39.92000:08:39.930 types of systems which occupy more space 00:08:42.23000:08:42.240 are generally used in smaller capacity 00:08:44.69000:08:44.700 systems okay now let us look at the next 00:08:48.05000:08:48.060 type this is what is known as body load 00:08:50.81000:08:50.820 type of 00:08:51.41000:08:51.420 Reiter's this kind of an evaporator 00:08:55.10000:08:55.110 consists of a large number of horizontal 00:08:56.93000:08:56.940 pipes stacked one on top of other and 00:08:59.57000:08:59.580 connected together by headers to make 00:09:02.27000:09:02.280 singular multiple circuits the 00:09:05.63000:09:05.640 refrigerant is circulated inside the 00:09:07.25000:09:07.260 tubes either in flooded or dry mode that 00:09:09.77000:09:09.780 means you can have flooded evaporator or 00:09:11.75000:09:11.760 dry expansion type evaporator the liquid 00:09:16.46000:09:16.470 to be chilled flows in a thin layer over 00:09:18.89000:09:18.900 the outer surface of the tubes the 00:09:22.31000:09:22.320 liquid flows down by gravity from 00:09:23.78000:09:23.790 distributed pipe located on top of the 00:09:25.94000:09:25.950 horizontal tubes now let me show the 00:09:27.74000:09:27.750 picture of Barlow type of evaporator so 00:09:36.47000:09:36.480 this is the header okay so this is one 00:09:40.25000:09:40.260 header on this side and this is another 00:09:43.13000:09:43.140 header header on the refrigerant Inlet 00:09:45.14000:09:45.150 and your Inlet side okay this is the 00:09:47.09000:09:47.100 header on the outlet side so to this 00:09:49.16000:09:49.170 header a large number of refrigerant 00:09:51.23000:09:51.240 tubes are connected these are the 00:09:52.61000:09:52.620 refrigerant tubes okay refrigerant tubes 00:09:58.27000:09:58.280 through which the refrigerant flows so 00:10:01.07000:10:01.080 the refrigerant enters into the header 00:10:03.17000:10:03.180 and from the header it enters into the 00:10:05.45000:10:05.460 parallel several parallel tubes okay and 00:10:08.48000:10:08.490 it extracts heat from the cool liquid 00:10:11.06000:10:11.070 being chilled and it exists from this 00:10:13.04000:10:13.050 header okay so from this header it goes 00:10:15.71000:10:15.720 to the compressor now the liquid being 00:10:18.47000:10:18.480 chilled for example milk it enters into 00:10:21.32000:10:21.330 the header which is kept at the top okay 00:10:23.45000:10:23.460 you can call this as header or 00:10:24.86000:10:24.870 distributor this has several nozzles 00:10:26.96000:10:26.970 okay you can see that these are all the 00:10:28.51000:10:28.520 nozzles are okay through these nozzles 00:10:35.54000:10:35.550 the liquid that is being chilled falls 00:10:38.72000:10:38.730 onto these refrigerant tubes okay 00:10:41.27000:10:41.280 because of gravity so gravity is in this 00:10:42.95000:10:42.960 direction so liquid falls on the tubes 00:10:45.11000:10:45.120 and it trickles down right and finally 00:10:49.07000:10:49.080 it gets collected in the bottom tank 00:10:51.20000:10:51.210 right the chilled milk or chilled water 00:10:53.99000:10:54.000 gets collected in this bottom tank okay 00:10:58.48000:10:58.490 from the bottom tank it is taken out 00:11:00.83000:11:00.840 right so if you look at a single tube 00:11:03.92000:11:03.930 you will find that you have 00:11:05.21000:11:05.220 a single tube like this okay this is the 00:11:09.35000:11:09.360 wall of the tube through this tube 00:11:11.54000:11:11.550 refrigerant is flowing and outside you a 00:11:14.03000:11:14.040 thin layer of liquid gets collected okay 00:11:18.71000:11:18.720 outside you have liquid layer okay this 00:11:24.67900:11:24.689 liquid is the liquid that is being 00:11:25.97000:11:25.980 chilled so since this has a small 00:11:28.79000:11:28.800 thickness the heat transfer will be 00:11:30.92000:11:30.930 quite effective here okay so this is 00:11:33.35000:11:33.360 known as bordello type evaporator so the 00:11:42.11000:11:42.120 as you can see that the liquid to be 00:11:44.09000:11:44.100 chilled is open to atmosphere that means 00:11:46.00900:11:46.019 it is at atmospheric pressure hence its 00:11:48.71000:11:48.720 aeration may take place during cooling 00:11:52.36000:11:52.370 bottle evaporator is widely used for 00:11:54.82900:11:54.839 cooling milk or cooling wine and for 00:11:56.96000:11:56.970 chilling water for carbonation in 00:11:58.87900:11:58.889 bottling plants one advantage of this 00:12:02.60000:12:02.610 kind of an evaporator is that the liquid 00:12:04.79000:12:04.800 can be chilled very close to its 00:12:06.29000:12:06.300 freezing temperature since freezing 00:12:07.87900:12:07.889 outside the tubes will not damage the 00:12:09.88900:12:09.899 tubes okay so this is one of the 00:12:11.86000:12:11.870 advantages of bottler type of evaporator 00:12:16.78000:12:16.790 next let us look at the direct expansion 00:12:19.18900:12:19.199 plate fin and tube type evaporator this 00:12:21.47000:12:21.480 as the name implies the plate fin and 00:12:23.17900:12:23.189 tube type evaporator is constructional 00:12:25.18900:12:25.199 II similar to plate fin and tube type 00:12:27.61900:12:27.629 condenser okay so you hear the external 00:12:30.49900:12:30.509 fluid is a right incase of condenser 00:12:33.67900:12:33.689 also the external fluid is air and the 00:12:36.04900:12:36.059 refrigerant flows through the tubes so 00:12:37.67000:12:37.680 in case of plate fin and tube type of 00:12:39.23000:12:39.240 evaporator also the refrigerant flows 00:12:41.36000:12:41.370 through the tubes while the air flows 00:12:43.49000:12:43.500 outside the tubes okay 00:12:47.13900:12:47.149 these evaporators are used for cooling 00:12:49.67000:12:49.680 and unifying the air directly by the 00:12:51.79900:12:51.809 refrigerant flowing in the tubes the 00:12:54.88900:12:54.899 liquid refrigerant enters from top 00:12:56.50900:12:56.519 through a thermostatic expansion valve 00:12:59.35000:12:59.360 this arrangement makes the oil returned 00:13:01.87900:13:01.889 to compressor better rather than feeding 00:13:03.55900:13:03.569 refrigerant from the bottom of the coil 00:13:05.56900:13:05.579 then the fin spacing varies from 50 to 00:13:09.25900:13:09.269 500 fins per meter length so let me show 00:13:12.29000:13:12.300 the picture of this so you can see that 00:13:14.43900:13:14.449 construction Lee this is almost similar 00:13:16.46000:13:16.470 to your plate fin and tube type 00:13:17.99000:13:18.000 condenser 00:13:18.98000:13:18.990 okay so the refrigerant which flows 00:13:21.32000:13:21.330 through this tube enters at the top 00:13:23.38000:13:23.390 right it flows like this through the 00:13:25.85000:13:25.860 tube okay and it leaves the evaporator 00:13:31.25000:13:31.260 at the bottom of course you can have 00:13:33.17000:13:33.180 several rows of such tube that means you 00:13:36.32000:13:36.330 can have one row behind the other and 00:13:38.21000:13:38.220 outside you have the fins and these fins 00:13:40.43000:13:40.440 remember their plate fins okay that 00:13:42.41000:13:42.420 means if you look from the side you have 00:13:44.39000:13:44.400 the fin and these are the tubes okay so 00:13:50.12000:13:50.130 this is the side view if you see from 00:13:51.98000:13:51.990 the side and here the air which flows 00:13:57.29000:13:57.300 over the tubes and fins it gets cooled 00:14:01.24000:14:01.250 as it comes in contact with the 00:14:03.53000:14:03.540 evaporator surface and depending upon 00:14:05.90000:14:05.910 the surface temperature it may also get 00:14:08.08000:14:08.090 dehumidified there okay that means both 00:14:11.63000:14:11.640 the temperature as well as the moisture 00:14:13.34000:14:13.350 content of eight may reduce as the air 00:14:15.68000:14:15.690 flow through flows through this kind of 00:14:17.57000:14:17.580 an evaporator okay and as I said 00:14:23.48000:14:23.490 normally the fringe spacing varies from 00:14:25.31000:14:25.320 50 to 500 fins per meter length and 00:14:29.42000:14:29.430 evaporators the atmospheric water vapor 00:14:31.28000:14:31.290 condenses on the fins and tubes when the 00:14:33.56000:14:33.570 metal temperature is lower than dew 00:14:35.12000:14:35.130 point temperature actually this is one 00:14:36.80000:14:36.810 reason why the design of evaporators 00:14:40.28000:14:40.290 that especially where air is external 00:14:43.52000:14:43.530 fluid is quite complicated compared to 00:14:46.28000:14:46.290 condensers because when the refrigerant 00:14:49.22000:14:49.230 temperature is low and when the surface 00:14:51.32000:14:51.330 of the evaporator is rated at a 00:14:53.12000:14:53.130 temperature lower than the corresponding 00:14:55.22000:14:55.230 dew point temperature of air then the 00:14:57.32000:14:57.330 moisture in the air will condense that 00:14:59.06000:14:59.070 means on the evaporator surface you have 00:15:01.19000:15:01.200 both heat as well as mass transfers okay 00:15:03.92000:15:03.930 so in the design of the evaporator is 00:15:05.30000:15:05.310 you have to consider both heat transfer 00:15:07.10000:15:07.110 rate as well as mass transfer rate the 00:15:09.11000:15:09.120 mass transfer rate will give rise to 00:15:10.58000:15:10.590 latent heat transfer rate that means 00:15:12.47000:15:12.480 both sensible as well as latent heat 00:15:14.51000:15:14.520 transfer rates have got to be considered 00:15:15.95000:15:15.960 while designing the evaporators for 00:15:19.22000:15:19.230 cooling and dehumidifying applications 00:15:21.05000:15:21.060 and as we have discussed in our earlier 00:15:23.03000:15:23.040 lectures cooling and dehumidification is 00:15:25.52000:15:25.530 mainly required in air conditioning 00:15:27.35000:15:27.360 plants of course in some of the cold 00:15:29.30000:15:29.310 storages also cooling and humidification 00:15:30.86000:15:30.870 is required okay so 00:15:32.75000:15:32.760 one of the common requirements and as I 00:15:35.72000:15:35.730 said the difference between a plate fin 00:15:38.30000:15:38.310 and tube type evaporator and condenser 00:15:40.31000:15:40.320 is that in the condenser a does not 00:15:42.65000:15:42.660 undergo any latent heat transfer whereas 00:15:44.48000:15:44.490 in the evaporator both sensible as well 00:15:46.40000:15:46.410 as latent heat transfer takes place on 00:15:48.11000:15:48.120 the air side also okay and if the 00:15:52.91000:15:52.920 surface temperature is less than zero 00:15:54.62000:15:54.630 degree centigrade then frost also may 00:15:56.38900:15:56.399 form that means whatever liquid 00:15:57.98000:15:57.990 condenses it will also freeze if the 00:16:00.59000:16:00.600 surface temperature is lower than zero 00:16:02.66000:16:02.670 degree centigrade this happens in 00:16:05.00000:16:05.010 domestic refrigerators and all you must 00:16:06.74000:16:06.750 have seen the evaporator of domestic 00:16:08.75000:16:08.760 refrigerator being covered with a thick 00:16:10.34000:16:10.350 layer of frost because the evaporator 00:16:12.53000:16:12.540 operates at a temperature much lower 00:16:14.48000:16:14.490 than zero degree centigrade in a 00:16:16.18900:16:16.199 domestic refrigerator hence for low 00:16:19.75900:16:19.769 temperature coils so what is the 00:16:21.74000:16:21.750 consequence of this we have to consider 00:16:23.90000:16:23.910 this one while designing so what is done 00:16:26.00000:16:26.010 is for low temperature coils that means 00:16:28.18900:16:28.199 a coils which operate at low 00:16:30.29000:16:30.300 temperatures such that frosting may take 00:16:33.41000:16:33.420 place avoid finishing that means about 00:16:36.07900:16:36.089 80 to 200 fins per meter is used to 00:16:39.17000:16:39.180 avoid blockage of flow passes due to 00:16:41.32900:16:41.339 frost formation that means you cannot 00:16:43.43000:16:43.440 afford to have very close fins when 00:16:47.05900:16:47.069 there is a possibility of frosting 00:16:49.12900:16:49.139 taking place on the outside okay if the 00:16:51.41000:16:51.420 fin spacing is very close then if frost 00:16:53.96000:16:53.970 forms then the frost will very soon 00:16:55.96000:16:55.970 block the it passages okay once the my 00:16:59.66000:16:59.670 passes are blocked air cannot flow okay 00:17:02.75000:17:02.760 so that is the reason why in evaporator 00:17:06.16900:17:06.179 is used for domestic refrigerators or in 00:17:08.05900:17:08.069 cold storages where frosting takes place 00:17:10.37000:17:10.380 the fin spacing has got to be wider so 00:17:13.37000:17:13.380 as I have mentioned in an earlier slide 00:17:14.72000:17:14.730 the fin spacing is normally between 50 00:17:16.85000:17:16.860 to 500 fins per meter as long as there 00:17:20.27000:17:20.280 is no frost formation okay the frost 00:17:21.91900:17:21.929 formation is there then the fin spacing 00:17:23.84000:17:23.850 varies from about 80 to 200 fins per 00:17:26.65900:17:26.669 meter and then the commercial 00:17:29.00000:17:29.010 evaporators you will find that the fin 00:17:30.91900:17:30.929 spacing or may not be uniform okay 00:17:33.40900:17:33.419 especially if the fin is not plate fin 00:17:35.53900:17:35.549 tube type okay in a typical in domestic 00:17:38.03000:17:38.040 refrigerators and all the fins are 00:17:40.13000:17:40.140 provided in asada in such a way that at 00:17:42.02000:17:42.030 the inlet where the frost formation is 00:17:44.06000:17:44.070 much faster the fin spacing is high 00:17:46.52000:17:46.530 okay and as the a gradually moves up the 00:17:50.06000:17:50.070 fringe spacing is gradually reduced so 00:17:52.34000:17:52.350 that you can have higher heat transfer 00:17:54.20000:17:54.210 okay so this is a very important aspect 00:17:56.42000:17:56.430 to be considered in the design of 00:17:58.46000:17:58.470 evaporators which are likely to get 00:18:00.98000:18:00.990 frosted okay and the since so you cannot 00:18:06.13000:18:06.140 allow the frosting to take place 00:18:08.27000:18:08.280 continuously frequent frequent 00:18:10.04000:18:10.050 defrosting is required in air 00:18:14.63000:18:14.640 conditioning applications a typical 00:18:16.22000:18:16.230 fringe spacing of 1.8 mm is used that 00:18:18.89000:18:18.900 means in air conditioning applications 00:18:19.88000:18:19.890 the fin spacing can be slightly higher 00:18:22.52000:18:22.530 compared to the evaporator used in 00:18:25.01000:18:25.020 domestic refrigerators because in air 00:18:26.84000:18:26.850 conditioning applications water 00:18:28.85000:18:28.860 condenses but it does not form frost if 00:18:32.33000:18:32.340 it forms frost then frost cannot leave 00:18:34.52000:18:34.530 the evaporator surface okay it sticks to 00:18:37.13000:18:37.140 the evaporator surface then it is a it 00:18:39.08000:18:39.090 starts blocking the airflow passage 00:18:41.69000:18:41.700 whereas if water only condenses then due 00:18:44.54000:18:44.550 to gravity it can trickle down okay it 00:18:46.31000:18:46.320 will not affect the air flow passage 00:18:48.17000:18:48.180 that is reason why in air conditioning 00:18:49.82000:18:49.830 applications the fin spacing is much 00:18:51.68000:18:51.690 smaller compared to a domestic frost 00:18:54.83000:18:54.840 free evaporator okay now let us look at 00:19:00.83000:19:00.840 plate surface evaporators plate surface 00:19:05.18000:19:05.190 tur one type of plate surface evaporator 00:19:07.22000:19:07.230 is also known as roll bond type 00:19:08.57000:19:08.580 evaporator and this is normally used in 00:19:11.03000:19:11.040 conventional domestic refrigerators okay 00:19:13.94000:19:13.950 that means not automatic refills but or 00:19:16.36000:19:16.370 refrigerators in which the frosting has 00:19:18.35000:19:18.360 to be done manually okay so let me show 00:19:20.84000:19:20.850 the picture of a roll bound type 00:19:22.43000:19:22.440 evaporator this is a roll bound type 00:19:26.29000:19:26.300 evaporator okay so it basically consists 00:19:30.83000:19:30.840 of a plate which consists of revver 00:19:32.90000:19:32.910 refrigerant tubing okay so this is 00:19:34.79000:19:34.800 refrigerant passages I should not call 00:19:37.01000:19:37.020 it a tubing but this is refrigerant 00:19:38.81000:19:38.820 passage okay through which the 00:19:43.67000:19:43.680 refrigerant flows for example 00:19:44.69000:19:44.700 refrigerant enters like this flows this 00:19:47.51000:19:47.520 way okay and finally leaves like this 00:19:49.73000:19:49.740 and air flows over the outside that 00:19:52.43000:19:52.440 means on this surface you have a and if 00:19:55.82000:19:55.830 you take a section at let us say at a 00:19:57.71000:19:57.720 the section looks like this 00:19:59.64000:19:59.650 okay so this is your plate okay and this 00:20:05.64000:20:05.650 is your refrigerator okay and eight is 00:20:09.21000:20:09.220 on the outside so the manufacturing of 00:20:13.44000:20:13.450 this is quite interesting what is the 00:20:15.45000:20:15.460 generally done is to aluminum plates of 00:20:20.33000:20:20.340 equal size that means equal thickness 00:20:23.10000:20:23.110 equal width etcetera are taken and on 00:20:25.29000:20:25.300 these aluminum plates the refrigerant 00:20:27.29900:20:27.309 circuit is printed that means it's just 00:20:28.95000:20:28.960 like your PCB okay printed circuit board 00:20:31.23000:20:31.240 the refrigerant tubing circuit is 00:20:33.00000:20:33.010 printed on this printer printing means 00:20:35.37000:20:35.380 some ink is deposited in the way the 00:20:37.98000:20:37.990 tubing is required okay so once the 00:20:40.38000:20:40.390 printing is done both these aluminum 00:20:42.24000:20:42.250 plates are brought together and both the 00:20:45.33000:20:45.340 plates are sent to rolling mills okay so 00:20:47.88000:20:47.890 both the plates are rolled in rolling 00:20:50.10000:20:50.110 mills so when the plates are rolled in 00:20:52.23000:20:52.240 let us say hot rolling mills then 00:20:54.39000:20:54.400 bonding of the two plates takes place 00:20:56.07000:20:56.080 that means both the power plates get 00:20:58.56000:20:58.570 fused together and they form a a single 00:21:00.72000:21:00.730 plate this bonding takes place 00:21:02.61000:21:02.620 everywhere except where there is ink 00:21:05.19000:21:05.200 okay so wherever there is ink the 00:21:07.26000:21:07.270 bonding does not take place okay that is 00:21:08.97000:21:08.980 the characteristic of the ink used okay 00:21:12.39000:21:12.400 so after the rolling process what is 00:21:14.58000:21:14.590 done is the refrigerant passage that 00:21:17.43000:21:17.440 means wherever ink is there is insulated 00:21:19.79900:21:19.809 by sending high pressure water or high 00:21:21.96000:21:21.970 pressure air okay since there is no 00:21:24.27000:21:24.280 bonding wherever there is ink at that 00:21:27.29900:21:27.309 point under high pressure of the tubes I 00:21:31.89000:21:31.900 mean the plate can get inflated okay 00:21:34.14000:21:34.150 that means plates remain separate only 00:21:36.41900:21:36.429 where there is ink rest of the place 00:21:38.28000:21:38.290 they get used okay since they are 00:21:40.53000:21:40.540 separate where ink is there they can be 00:21:42.72000:21:42.730 insulated by sending high-pressure fluid 00:21:44.85000:21:44.860 okay so once the high-pressure fluid is 00:21:47.82000:21:47.830 sent the refrigerant flow passage is 00:21:49.71000:21:49.720 formed okay so once the passage is 00:21:51.81000:21:51.820 formed the high pressure is withdrawn 00:21:53.04000:21:53.050 then it is clean right so what you have 00:21:55.44000:21:55.450 is a single plate okay which is made of 00:21:58.68000:21:58.690 two separate plates a single plate 00:22:01.11000:22:01.120 consisting of inbuilt refrigerant 00:22:03.51000:22:03.520 passages through which refrigerant can 00:22:05.31000:22:05.320 flow okay so that is what is shown here 00:22:07.53000:22:07.540 you have the finally when everything is 00:22:10.14000:22:10.150 over at this point the inlet and outlet 00:22:12.75000:22:12.760 two 00:22:13.42000:22:13.430 are welded to the Roll Bond type 00:22:16.00000:22:16.010 evaporator okay so through this tube 00:22:17.77000:22:17.780 refrigerant enters and through from this 00:22:20.14000:22:20.150 it comes out and this plate can be bent 00:22:22.81000:22:22.820 into any form for example this plate can 00:22:24.88000:22:24.890 be made into the form of a box or it can 00:22:28.36000:22:28.370 be made into the form of Cu it can be 00:22:29.95000:22:29.960 bent into the form of L right so it can 00:22:32.23000:22:32.240 be bend into any shape and this is 00:22:34.96000:22:34.970 nothing but your evaporator and it also 00:22:38.89000:22:38.900 is nothing but your freezer box this is 00:22:41.11000:22:41.120 what you see in a conventional domestic 00:22:43.39000:22:43.400 refrigerator the moment you open the 00:22:45.40000:22:45.410 door you find the freezer box okay and 00:22:47.98000:22:47.990 you will also notice that the freezer 00:22:49.57000:22:49.580 box consists of some protrusions okay 00:22:52.54000:22:52.550 and freezer box made of aluminium plate 00:22:54.55000:22:54.560 and it has some protrusion that is 00:22:56.32000:22:56.330 nothing but your refrigerant flow 00:22:57.94000:22:57.950 passage through which the refrigerant 00:22:59.02000:22:59.030 flows okay this kind of evaporator is 00:23:01.96000:23:01.970 very very effective and it is also 00:23:03.85000:23:03.860 easier to manufacture and it is cost 00:23:07.33000:23:07.340 wise also it is good it is cheaper 00:23:09.25000:23:09.260 compared to the cube and plate type 00:23:11.11000:23:11.120 evaporator okay that is the reason why 00:23:12.88000:23:12.890 most of the manufacturers nowadays use 00:23:15.40000:23:15.410 this kind of evaporators in domestic 00:23:17.38000:23:17.390 refrigerators okay I in addition to that 00:23:20.95000:23:20.960 because very good bonding the heat 00:23:23.23000:23:23.240 transfer is very effective okay and if 00:23:25.69000:23:25.700 you look at any single tube you find 00:23:27.34000:23:27.350 that this area acts as a fin okay under 00:23:33.25000:23:33.260 the contact resistance between the fin 00:23:34.81000:23:34.820 and the refrigerant is almost negligible 00:23:37.72000:23:37.730 okay because they are formed of the same 00:23:39.46000:23:39.470 plate right so these are the advantages 00:23:42.25000:23:42.260 of roll bound type of evaporators which 00:23:43.99000:23:44.000 are used in conventional domestic 00:23:46.15000:23:46.160 refrigerators and other applications 00:23:50.52000:23:50.530 there is another type of plate surface 00:23:53.17000:23:53.180 evaporator in this type a serpentine 00:23:56.14000:23:56.150 tube is placed between two metal plates 00:23:58.24000:23:58.250 such that the plates press on to the 00:24:00.43000:24:00.440 tube okay so let me just describe it 00:24:04.93000:24:04.940 first the edges of the plates are welded 00:24:06.88000:24:06.890 together the space between the plates is 00:24:09.52000:24:09.530 either filled with a eutectic solution 00:24:10.96000:24:10.970 or evacuated if eutectic solution is 00:24:14.05000:24:14.060 provided it provides a good hold up 00:24:15.91000:24:15.920 capacity and this kind of evaporators 00:24:20.47000:24:20.480 are widely used and refrigerate 00:24:21.94000:24:21.950 refrigerated trucks 00:24:24.09000:24:24.100 can you describe this so you can see 00:24:26.67000:24:26.680 here okay and this is the 00:24:33.33000:24:33.340 cross-sectional view at a right so this 00:24:36.78000:24:36.790 is like a closed box at a closed box in 00:24:41.97000:24:41.980 which the refrigerant tubes okay the 00:24:45.09000:24:45.100 serpentine tube is sandwiched right that 00:24:47.64000:24:47.650 mean you take two plates and put this 00:24:49.80000:24:49.810 serpentine coil and close all the wedges 00:24:52.83000:24:52.840 right and have the inlet and outlet term 00:24:55.80000:24:55.810 right now you have some space between 00:24:58.92000:24:58.930 the refrigerant this is the refrigerant 00:25:01.02000:25:01.030 tube okay these are the refrigerant 00:25:02.43000:25:02.440 tubes and this is the plate over which 00:25:04.80000:25:04.810 your air flows air or whatever is the 00:25:06.66000:25:06.670 external fluid that flows right so there 00:25:09.09000:25:09.100 is a gap between the tube and the plate 00:25:12.18000:25:12.190 which is all the space so that space is 00:25:16.05000:25:16.060 normally filled with a eutectic solution 00:25:18.12000:25:18.130 okay eutectic solution means a solution 00:25:20.04000:25:20.050 which has low freezing point right so 00:25:22.53000:25:22.540 when you fill this space with the 00:25:23.76000:25:23.770 eutectic solution that means this space 00:25:25.35000:25:25.360 then there is a good thermal contact 00:25:28.38000:25:28.390 between the refrigerant tube and the 00:25:31.20000:25:31.210 outer plate okay because the eutectic 00:25:33.30000:25:33.310 solution which is a liquid has a high 00:25:35.58000:25:35.590 thermal conductivity compared to a 00:25:37.14000:25:37.150 vacuum as a result you get good 00:25:39.75000:25:39.760 performance okay in another method what 00:25:42.81000:25:42.820 is done is this space is not filled with 00:25:45.96000:25:45.970 any solution but it is evacuated okay 00:25:48.54000:25:48.550 when you evacuate it the outside 00:25:50.19000:25:50.200 pressure which is atmospheric that 00:25:52.02000:25:52.030 presses these two outer tubes as a 00:25:54.36000:25:54.370 result again good thermal contact is 00:25:56.40000:25:56.410 formed between the tubes and the plates 00:25:58.56000:25:58.570 right but more widely the eutectic 00:26:01.41000:26:01.420 solution is used because the advantage 00:26:03.09000:26:03.100 of eutectic solution is that in addition 00:26:05.34000:26:05.350 to providing good thermal contact it 00:26:07.47000:26:07.480 also provides what is known as holdup 00:26:09.51000:26:09.520 capacity that means when you have lot of 00:26:12.29000:26:12.300 solution in the evaporator okay lot of 00:26:15.57000:26:15.580 external fluid in the evaporator the 00:26:17.49000:26:17.500 external fluid has since it is a liquid 00:26:19.80000:26:19.810 it has large specific heat right and it 00:26:23.04000:26:23.050 also has large mass because of its high 00:26:25.14000:26:25.150 density okay so you have large MCP 00:26:27.51000:26:27.520 liquid which is cooled to almost the 00:26:30.18000:26:30.190 same temperature and few degrees higher 00:26:32.70000:26:32.710 than the evaporator temperature okay so 00:26:34.98000:26:34.990 that means entire you have a PAC 00:26:36.77000:26:36.780 or you have an evaporator which has high 00:26:38.87000:26:38.880 large thermal capacity and which is 00:26:40.91000:26:40.920 cooled to a low temperature okay now let 00:26:43.49000:26:43.500 us say that there is a power cut okay so 00:26:45.62000:26:45.630 there is a power cut then the 00:26:46.85000:26:46.860 refrigeration is not provided reference 00:26:48.56000:26:48.570 system does not work so no refrigeration 00:26:50.45000:26:50.460 is provided because the compressor does 00:26:52.01000:26:52.020 not work but still because of the large 00:26:53.93000:26:53.940 holdup capacity that means because of 00:26:55.61000:26:55.620 the large amount of liquid which is 00:26:57.62000:26:57.630 originally present in the evaporator and 00:26:59.51000:26:59.520 which was at very low temperature the 00:27:01.55000:27:01.560 refrigerated space can be maintained 00:27:03.59000:27:03.600 colder for a longer time okay that means 00:27:06.37000:27:06.380 this liquid the eutectic solution 00:27:09.35000:27:09.360 continuously gives cooling as long as 00:27:12.35000:27:12.360 its temperature is low okay so this kind 00:27:14.51000:27:14.520 of capacity is known as holdup capacity 00:27:16.58000:27:16.590 okay so this is one of the advantage of 00:27:18.20000:27:18.210 having this eutectic solution okay it 00:27:20.09000:27:20.100 can take care of power occurs or load 00:27:23.15000:27:23.160 variations etcetera okay and this is 00:27:25.55000:27:25.560 very widely used in refrigerated trucks 00:27:27.73000:27:27.740 what is done in a refrigerated truck is 00:27:30.20000:27:30.210 these evaporators are connected to 00:27:32.72000:27:32.730 refrigeration systems which are on the 00:27:34.40000:27:34.410 land 00:27:34.85000:27:34.860 okay so just before the truck starts all 00:27:37.91000:27:37.920 these evaporators are cooled to the 00:27:39.68000:27:39.690 required temperature that means the 00:27:41.66000:27:41.670 eutectic solution is cool to the 00:27:43.91000:27:43.920 required low temperature okay then the 00:27:46.37000:27:46.380 evaporators are disconnected from the 00:27:47.90000:27:47.910 refrigeration system and they are placed 00:27:49.64000:27:49.650 in the refrigerated truck okay then the 00:27:52.28000:27:52.290 truck moves from one phase to the other 00:27:53.66000:27:53.670 so as long as the truck is on the move 00:27:56.12000:27:56.130 cooling is provided by the cold solution 00:27:58.94000:27:58.950 placed in the evaporators okay so that 00:28:04.25000:28:04.260 way this simple system and evaporation 00:28:06.77000:28:06.780 is our I am sorry cooling is ensured 00:28:08.54000:28:08.550 even when the truck is moving without 00:28:10.61000:28:10.620 putting any refrigerant system on board 00:28:12.83000:28:12.840 okay so this is one of the popular ways 00:28:15.38000:28:15.390 of providing refrigeration okay 00:28:24.77000:28:24.780 now let us look at another very 00:28:27.09000:28:27.100 important type of evaporator what is 00:28:28.89000:28:28.900 known as plate type evaporator plate 00:28:31.35000:28:31.360 type evaporators are used when it closed 00:28:34.29000:28:34.300 temperature approach that means 0.5 00:28:37.23000:28:37.240 kelvin or less between the boiling 00:28:39.24000:28:39.250 refrigerant and the fluid being chilled 00:28:41.22000:28:41.230 is required okay let me let us say that 00:28:43.91000:28:43.920 refrigerant is at minus 20 degree 00:28:46.23000:28:46.240 centigrade and I want to chill the 00:28:47.70000:28:47.710 external fluid to minus 19 point five 00:28:49.98000:28:49.990 degrees okay that means refrigerant 00:28:52.11000:28:52.120 temperature and the external fluid 00:28:54.06000:28:54.070 temperature are almost same okay the 00:28:56.76000:28:56.770 difference is very small about 0.5 00:28:58.29000:28:58.300 Kelvin then you require very high heat 00:29:00.51000:29:00.520 transfer coefficients in Donlon okay and 00:29:03.21000:29:03.220 you also require basically very high 00:29:05.94000:29:05.950 heat transfer area also right in such 00:29:08.40000:29:08.410 cases the plate type evaporators are 00:29:10.89000:29:10.900 used okay that means wherever you want 00:29:12.90000:29:12.910 very close approach plate type 00:29:14.61000:29:14.620 evaporators are used these evaporators 00:29:19.62000:29:19.630 are widely used in dairy plants for 00:29:21.54000:29:21.550 chilling milk in brewery for chilling 00:29:23.79000:29:23.800 beer etcetera these evaporators consist 00:29:28.05000:29:28.060 of a series of plates normally made of 00:29:30.24000:29:30.250 stainless steel between which 00:29:32.25000:29:32.260 alternately the milk or beer to be 00:29:34.92000:29:34.930 cooled and refrigerant flow in counter 00:29:37.14000:29:37.150 flow direction so let me show the 00:29:38.97000:29:38.980 picture of a plate type evaporator so 00:29:45.21000:29:45.220 you can see here that there are a large 00:29:47.40000:29:47.410 number of tubes for example this I mean 00:29:49.35000:29:49.360 large number of plates sorry you have 00:29:51.33000:29:51.340 this is one plate and this is another 00:29:53.88000:29:53.890 plate okay so like that a large number 00:29:56.73000:29:56.740 of plates are stacked together okay and 00:30:00.56000:30:00.570 the flow passes is there between these 00:30:04.71000:30:04.720 two plates okay that means there is some 00:30:06.93000:30:06.940 gap between these two plates through 00:30:08.40000:30:08.410 which the fluid can flow right for 00:30:10.89000:30:10.900 example let us say that here the 00:30:13.17000:30:13.180 refrigerant is flowing in this direction 00:30:14.54000:30:14.550 okay so in the passage formed by this 00:30:17.37000:30:17.380 plate and the next plate that means in 00:30:19.29000:30:19.300 this passage the liquid being cool will 00:30:23.10000:30:23.110 be flowing in the opposite direction 00:30:24.33000:30:24.340 that means one fluid flows in this 00:30:26.16000:30:26.170 direction the other fluid flows in this 00:30:28.29000:30:28.300 direction okay so in the next passage 00:30:30.69000:30:30.700 that means the path is found between 00:30:32.10000:30:32.110 displayed and the next plate again the 00:30:34.16900:30:34.179 refrigerant flowing will be flowing in 00:30:35.76000:30:35.770 this direction 00:30:36.81000:30:36.820 okay that means if you look at the 00:30:40.62000:30:40.630 plates let us say that these are the 00:30:42.06000:30:42.070 number of plates which are stacked 00:30:43.28900:30:43.299 together if the refrigerant will be 00:30:45.09000:30:45.100 flowing like this okay in alternate 00:30:47.61000:30:47.620 paths and the liquid being cooled will 00:30:50.63900:30:50.649 be flowing like this okay so this is 00:30:54.36000:30:54.370 this is refrigerant and this is the 00:30:56.37000:30:56.380 layer external fluid so you can see that 00:30:58.35000:30:58.360 between the passages a true counter flow 00:31:00.41900:31:00.429 can be obtained so as we know the 00:31:02.54900:31:02.559 counter flow graph gives rise to very 00:31:04.04900:31:04.059 effective heat transfer okay and you can 00:31:07.68000:31:07.690 also see that these plates are not plain 00:31:13.28900:31:13.299 plates but they have some profile on 00:31:16.11000:31:16.120 that normally they are they have some 00:31:20.61000:31:20.620 structure in such a way that a good 00:31:22.79900:31:22.809 turbulence and hence high heat transfer 00:31:25.23000:31:25.240 coefficients can be obtained okay 00:31:27.36000:31:27.370 normally having bone structure or 00:31:28.68000:31:28.690 something like that is used okay so the 00:31:30.69000:31:30.700 plane plates are taken and they are 00:31:32.36900:31:32.379 stamped with the required structure so 00:31:35.36900:31:35.379 that you get a high heat transfer 00:31:37.39900:31:37.409 coefficient okay so you have if you look 00:31:40.08000:31:40.090 at each plate you will have this kind of 00:31:42.68000:31:42.690 stamping and you can see that the 00:31:46.59000:31:46.600 refrigerant enters for example in this 00:31:48.21000:31:48.220 figure through the end plate it enters 00:31:52.08000:31:52.090 like this so flows through alternate 00:31:54.11900:31:54.129 passages okay and it comes out like this 00:31:57.89900:31:57.90900:31:59.27900:31:59.289 whereas water enters through this 00:32:01.95000:32:01.960 passage again flows through the 00:32:03.33000:32:03.340 evaporator and it comes out from this 00:32:08.24000:32:08.250 passage in the end plate okay and the 00:32:10.88900:32:10.899 ends are closed you have one end plate 00:32:12.57000:32:12.580 this is one end plate and this is end 00:32:14.22000:32:14.230 plate and when everything is put 00:32:16.13900:32:16.149 together we have you also require a 00:32:19.11000:32:19.120 gasket to prevent leakage for example 00:32:21.29900:32:21.309 this is a gasket so each plate will have 00:32:24.06000:32:24.070 gasket on both sides and the gasket will 00:32:26.00900:32:26.019 prevent leakage of refrigerant and also 00:32:28.95000:32:28.960 provide the gap for flow passage okay so 00:32:32.43000:32:32.440 when you stack the plates together and 00:32:35.49000:32:35.500 close it with the end plates then a 00:32:37.08000:32:37.090 closed evaporator is formed and the 00:32:41.03900:32:41.049 entire evaporator is assembled okay so 00:32:45.02900:32:45.039 you have the assembly tie rods and all 00:32:47.87900:32:47.889 that okay 00:32:49.82000:32:49.830 so finally when you assemble a tuba you 00:32:51.86000:32:51.870 get a very compact evaporator which has 00:32:54.52900:32:54.539 very high heat transfer area ok so high 00:32:58.13000:32:58.140 capacity right so this is what is known 00:33:01.00900:33:01.019 as your plate type of evaporator 00:33:02.33000:33:02.340 nowadays it is becoming popular 00:33:03.50000:33:03.510 especially in large systems and also in 00:33:05.99000:33:06.000 the very plants and all where milk is 00:33:08.81000:33:08.820 used the reason why it is becoming 00:33:10.82000:33:10.830 popular in dairy plants and all is that 00:33:13.14900:33:13.159 in dairy plants you have to clean the 00:33:15.91900:33:15.929 evaporators almost every day okay 00:33:18.08000:33:18.090 because once the milk flows through it 00:33:20.87000:33:20.880 for a certain certain amount of time 00:33:22.22000:33:22.230 cleaning is required so to have to 00:33:23.89900:33:23.909 maintain the highs in right and if you 00:33:26.14900:33:26.159 are using let us say about brighter like 00:33:27.91900:33:27.929 shell-and-tube type of evaporator 00:33:29.21000:33:29.220 cleaning is very difficult or it may 00:33:31.58000:33:31.590 take lot of time okay 00:33:33.32000:33:33.330 whereas in plate type evaporator since 00:33:35.41900:33:35.429 the way it is constructed and the phase 00:33:36.98000:33:36.990 since by the way it is assembled if you 00:33:39.40900:33:39.419 want to clean it you have to remove some 00:33:40.75900:33:40.769 bolts remove the end plate then all the 00:33:43.07000:33:43.080 plates are accessible okay so you can 00:33:44.75000:33:44.760 remove one by one plate and clean it and 00:33:46.39900:33:46.409 again stack them together and assemble 00:33:48.28900:33:48.299 the whole evaporator okay so the 00:33:50.64900:33:50.659 removing the plates and cleaning and 00:33:53.09000:33:53.100 again assembling them back does not take 00:33:55.07000:33:55.080 much time okay so this is very 00:33:57.25900:33:57.269 convenient in this kind of plants where 00:33:59.64900:33:59.659 hygiene is important cleaning is 00:34:01.70000:34:01.710 required frequent cleaning is required 00:34:03.27900:34:03.289 okay so that is why these are very ideal 00:34:06.68000:34:06.690 in addition to that if you want to 00:34:08.69000:34:08.700 increase the capacity of the evaporator 00:34:10.87900:34:10.889 or reduce the capacity of the evaporator 00:34:12.50000:34:12.510 all that you have to do is you have to 00:34:13.90900:34:13.919 remove the bolts take out one end plate 00:34:15.82900:34:15.839 and add few plates we want to increase 00:34:17.75000:34:17.760 the capacity or remove some plates we 00:34:19.66900:34:19.679 want to reduce the capacity okay so 00:34:21.53000:34:21.540 capacity reduction or increase is 00:34:23.59900:34:23.609 possible well very easily right that is 00:34:27.37900:34:27.389 the reason why these evaporators are 00:34:28.87900:34:28.889 becoming popular in addition to their 00:34:31.39900:34:31.409 being very very effective okay 00:34:42.29000:34:42.300 and the overall heat transfer 00:34:44.58000:34:44.590 coefficient in plate type evaporator is 00:34:46.92000:34:46.930 very large you get about 4500 watt per 00:34:49.65000:34:49.660 meter square Kelvin for ammonia water 00:34:51.48000:34:51.490 and 3,000 watt per meter squared Kelvin 00:34:54.03000:34:54.040 for r22 water them and these type of 00:34:59.55000:34:59.560 evaporators require very less 00:35:01.05000:35:01.060 refrigerant inventory for the same 00:35:03.18000:35:03.190 capacity okay that means the total 00:35:05.40000:35:05.410 amount of refrigerant to be charged into 00:35:07.23000:35:07.240 the system is much less when you are 00:35:09.87000:35:09.880 using plate type evaporator for example 00:35:11.79000:35:11.800 it will be about 10 percent of 00:35:13.65000:35:13.660 shell-and-tube type evaporators for same 00:35:16.44000:35:16.450 capacity okay so this is an advantage 00:35:18.21000:35:18.220 because it reduces the cost and if you 00:35:20.58000:35:20.590 are using some toxic refrigerant it is 00:35:22.44000:35:22.450 also safer to have less refrigerant and 00:35:26.33000:35:26.340 cleaning the evaporator in dairy plants 00:35:28.53000:35:28.540 and provirus is very easy as I have 00:35:30.00000:35:30.010 already explained the capacity can be 00:35:33.03000:35:33.040 increased or decreased very easily by 00:35:34.83000:35:34.840 adding or removing plates this also have 00:35:36.69000:35:36.700 explained okay so these are some of the 00:35:41.90000:35:41.910 important types of evaporators in fact 00:35:45.42000:35:45.430 if you look at operator unlike 00:35:46.98000:35:46.990 condensers there can be many a large 00:35:49.53000:35:49.540 variety of evaporators okay normally 00:35:51.96000:35:51.970 condensers can be made of the Shelf or 00:35:54.27000:35:54.280 you have standard types of condensers 00:35:56.97000:35:56.980 which can be made to order or which can 00:35:59.46000:35:59.470 be obtained off the shelf but evaporator 00:36:01.56000:36:01.570 generally are made to order okay most of 00:36:04.35000:36:04.360 the time the operators are non-standard 00:36:06.81000:36:06.820 depending upon the applications except 00:36:08.91000:36:08.920 for maybe a shell and tube type or a 00:36:12.47000:36:12.480 plate fin and tube type or a plate type 00:36:15.24000:36:15.250 okay the roll bond type of evaporators 00:36:17.58000:36:17.590 etc are made depending upon the 00:36:20.04000:36:20.050 requirement okay the design and 00:36:21.80000:36:21.810 manufacturing etcetera is done based on 00:36:23.88000:36:23.890 the specific requirement so you have a 00:36:25.74000:36:25.750 wide variety of evaporators okay 00:36:28.11000:36:28.120 depending upon a wide variety of 00:36:29.85000:36:29.860 applications okay this is one again the 00:36:32.13000:36:32.140 difference between evaporator and 00:36:33.69000:36:33.700 condenser okay now let us look at the 00:36:36.51000:36:36.520 thermal design of evaporators when I say 00:36:38.73000:36:38.740 thermal design of evaporators actually 00:36:40.29000:36:40.300 the design of evaporator is very very 00:36:41.73000:36:41.740 complicated 00:36:42.45000:36:42.460 unlike the design of condenser because 00:36:44.82000:36:44.830 of the factor that we have both we can 00:36:47.58000:36:47.590 have both the sensible as well as latent 00:36:49.23000:36:49.240 heat transfer taking place on the 00:36:51.24000:36:51.250 external fluid side also okay in 00:36:53.40000:36:53.410 addition to that the production of 00:36:55.56000:36:55.570 refrigerant coefficient on the boiling 00:36:58.98000:36:58.990 side is also quite difficult unlike that 00:37:01.74000:37:01.750 of condensation okay for this reason the 00:37:04.62000:37:04.630 very exact design of swap vertices quite 00:37:07.65000:37:07.660 difficult okay so there are large number 00:37:09.72000:37:09.730 of correlations available which are 00:37:12.00000:37:12.010 applicable to particular ranges or 00:37:15.21000:37:15.220 particular flow patterns etcetera so 00:37:17.79000:37:17.800 depending upon our the type of the 00:37:19.68000:37:19.690 evaporator that we are planning to use 00:37:20.97000:37:20.980 we have to use the suitable correlations 00:37:23.37000:37:23.380 okay so I will not really go into the 00:37:25.62000:37:25.630 details of the exact design of 00:37:27.96000:37:27.970 evaporators which you will normally 00:37:29.73000:37:29.740 study in an advanced course I simply 00:37:32.10000:37:32.110 explain the basic principles and the 00:37:35.00000:37:35.010 complexities to be considered while 00:37:36.90000:37:36.910 designing the evaporator okay first let 00:37:39.18000:37:39.190 us look at the complexities the design 00:37:42.51000:37:42.520 is complex due to a refrigerant side 00:37:45.51000:37:45.520 heat transfer coefficient varies widely 00:37:47.01000:37:47.020 along the length both sensible and 00:37:50.43000:37:50.440 latent heat transfer may take place on 00:37:51.90000:37:51.910 external fluid side as I've already told 00:37:54.20000:37:54.210 presence of lubricating oil in 00:37:56.31000:37:56.320 evaporator okay in condenser normally if 00:38:00.48000:38:00.490 it is a refrigerant is miscible with the 00:38:02.28000:38:02.290 oil then the presence of oil does not 00:38:05.31000:38:05.320 affect the heat transfer or pressure 00:38:07.29000:38:07.300 drop but what happens is when you come 00:38:09.42000:38:09.430 to the evaporator the oil tends to get 00:38:12.45000:38:12.460 separated from the evaporating 00:38:14.67000:38:14.680 refrigerant okay that means once the 00:38:16.68000:38:16.690 liquid starts boiling then lubricating 00:38:20.10000:38:20.110 oil gets separated from the refrigerant 00:38:22.95000:38:22.960 once it gets separated it tries to stick 00:38:25.44000:38:25.450 to the evaporator tube and it tries to 00:38:27.12000:38:27.130 settle down in the operator okay so this 00:38:30.39000:38:30.400 will complicate the design because you 00:38:32.19000:38:32.200 have to make sure that the refrigerant 00:38:34.20000:38:34.210 or lubricating oil is flowing back to 00:38:37.05000:38:37.060 the compressor okay to ensure that the 00:38:38.64000:38:38.650 compressor is operating properly 00:38:40.35000:38:40.360 okay so oil return has to be considered 00:38:43.23000:38:43.240 above while designing the operators in 00:38:45.27000:38:45.280 addition to that the presence of large 00:38:47.61000:38:47.620 amount of oil in the evaporator will 00:38:49.41000:38:49.420 also affect the heat transfer and 00:38:51.48000:38:51.490 pressure drop 00:38:52.64000:38:52.650 characteristics of the evaporator which 00:38:54.57000:38:54.580 need to be considered okay so this makes 00:38:57.09000:38:57.100 the design complicated and the 00:39:00.99000:39:01.000 evaporator pressure drop is more 00:39:02.46000:39:02.470 critical 00:39:03.67000:39:03.680 I mean this I have already explained in 00:39:04.99000:39:05.000 an earlier lecture compared to the 00:39:06.70000:39:06.710 pressure drop on the condenser side the 00:39:09.33900:39:09.349 pressure drop on the evaporator side has 00:39:11.44000:39:11.450 a more significant effect on the 00:39:12.84900:39:12.859 performance of the system okay so you 00:39:15.25000:39:15.260 have to design the operator in such a 00:39:16.87000:39:16.880 way that the pressure drop is less than 00:39:19.24000:39:19.250 the acceptable level okay that means it 00:39:21.30900:39:21.319 is as far as possible it should be as 00:39:22.87000:39:22.880 small as possible 00:39:24.33900:39:24.349 okay this again puts constraints on the 00:39:26.71000:39:26.720 design of the evaporator as I have 00:39:32.20000:39:32.210 already explained deferent velocity has 00:39:34.21000:39:34.220 to be optimized taking both oil Litton 00:39:36.88000:39:36.890 and pressure drop into account term and 00:39:40.02000:39:40.030 finally the part load operation may lead 00:39:42.46000:39:42.470 to flooding of the evaporator which may 00:39:44.74000:39:44.750 lead to slugging of the compressor 00:39:49.29000:39:49.300 estimation of heat transfer area and 00:39:51.30900:39:51.319 overall heat transfer coefficients for 00:39:54.57900:39:54.589 plate fin and tube type evaporators 00:39:56.29000:39:56.300 expressions for various areas are 00:39:58.00000:39:58.010 exactly same as that of plate fin and 00:39:59.71000:39:59.720 tube type condensers this we have 00:40:01.69000:40:01.700 discussed in detail while discussing the 00:40:03.33900:40:03.349 design of condensers so the how to 00:40:05.20000:40:05.210 calculate areas that is exactly same 00:40:06.97000:40:06.980 okay expressions for overall heat 00:40:11.10900:40:11.119 transfer coefficient is also similar as 00:40:12.94000:40:12.950 long as there is no latent heat transfer 00:40:14.74000:40:14.750 on eight side okay if there is no latent 00:40:17.23000:40:17.240 heat transfer on each side and if only 00:40:18.64000:40:18.650 sensible heat transfer takes place on 00:40:20.10900:40:20.119 the eight side then the design that 00:40:22.08900:40:22.099 means the expressions for you 00:40:23.14000:40:23.150 expressions for various area experience 00:40:25.69000:40:25.700 for fin efficiency etcetera they are 00:40:27.57900:40:27.589 exactly similar for a plate fin and tube 00:40:30.88000:40:30.890 type of evaporator as that of plate fin 00:40:33.06900:40:33.079 and type tip of condenser which we have 00:40:34.96000:40:34.970 already discussed write expressions for 00:40:39.28000:40:39.290 u LMT D and fin efficiency need to be 00:40:42.16000:40:42.170 modified if condensation or freezing 00:40:44.47000:40:44.480 takes place okay on the other hand if 00:40:46.27000:40:46.280 you have latent heat transfer on the 00:40:48.16000:40:48.170 external fluid side also then you have 00:40:50.17000:40:50.180 to consider these aspects also now let 00:40:54.81900:40:54.829 us look at estimation of heat transfer 00:40:56.23000:40:56.240 coefficients eight side heat transfer 00:40:59.34900:40:59.359 coefficients in plate fin and tube type 00:41:00.78900:41:00.799 evaporators and as I said these 00:41:02.79900:41:02.809 evaporators are normally used in larger 00:41:05.47000:41:05.480 in small air conditioning systems or in 00:41:08.07900:41:08.089 cold storages if air undergoes only 00:41:11.28900:41:11.299 sensible heat transfer correlations are 00:41:13.18000:41:13.190 same as that of air cooled condenser 00:41:15.19000:41:15.200 okay that means a 00:41:16.63000:41:16.640 there is only sensible heat transfer 00:41:17.58900:41:17.599 then you can use the same correlation 00:41:19.72000:41:19.730 that we have used for condensers namely 00:41:21.81900:41:21.829 for example in the cells correlation 00:41:23.58900:41:23.599 grim sense correlations etcetera mass 00:41:27.84900:41:27.859 transfer effects have to be considered 00:41:29.01900:41:29.029 if water vapor in air condenses or 00:41:31.56900:41:31.579 freezes 00:41:33.05900:41:33.069 hence analysis of cooling and 00:41:35.34900:41:35.359 humidification or cooling and freezing 00:41:37.00000:41:37.010 coils is more complicated due to 00:41:39.30900:41:39.319 simultaneous heat and mass transfer okay 00:41:41.85900:41:41.869 so to design cooling and 00:41:43.96000:41:43.970 dehumidification or cooling and freezing 00:41:45.40000:41:45.410 coils one has to know the mass transfer 00:41:48.81900:41:48.829 aspects also because you have to 00:41:50.50000:41:50.510 consider the simultaneous heat and mass 00:41:52.69000:41:52.700 transfer on the a side okay this is 00:41:55.39000:41:55.400 quite complex and normally this is 00:41:57.46000:41:57.470 started in advanced refrigeration 00:42:00.05900:42:00.069 courses okay now let us look at liquid 00:42:05.68000:42:05.690 side heat transfer coefficients liquid 00:42:09.40000:42:09.410 flowing in tubes 00:42:10.65000:42:10.660 when liquids such as water brine milk 00:42:13.29900:42:13.309 etcetera flow through tubes without 00:42:15.09900:42:15.109 undergoing any phase changes that means 00:42:16.69000:42:16.700 as long as it does not freeze it should 00:42:18.09900:42:18.109 not freeze but as long as it does not 00:42:19.69000:42:19.700 undergo any phase change the 00:42:21.24900:42:21.259 correlations presented earlier for 00:42:22.93000:42:22.940 condensers that means correlation such 00:42:25.12000:42:25.130 as detest bolter or sea death it can 00:42:27.37000:42:27.380 also be used for evaporator okay the 00:42:29.23000:42:29.240 exactly same correlations can be used 00:42:30.70000:42:30.710 with a small change in the deters 00:42:33.33900:42:33.349 bounder correlation for example the you 00:42:36.24900:42:36.259 have Prandtl number and the exponent of 00:42:37.99000:42:38.000 Prandtl number is 0.4 because there the 00:42:41.07900:42:41.089 liquid is being heated as it flows to 00:42:43.80900:42:43.819 the condenser right but when you use the 00:42:47.07900:42:47.089 same expression for evaporator for the 00:42:49.24000:42:49.250 external fluid then instead of using 00:42:51.06900:42:51.079 0.44 Prandtl number you have to take 0.3 00:42:54.00900:42:54.019 in detest builder correlation because 00:42:55.50900:42:55.519 the liquid is being cooled in case of 00:42:57.60900:42:57.619 evaporator this is the only change 00:42:59.14000:42:59.150 otherwise the correlation will be 00:43:00.46000:43:00.470 exactly same okay now let us look at the 00:43:06.64000:43:06.650 liquid flowing in a shell this will 00:43:08.38000:43:08.390 happen in shell and tube type of 00:43:09.99000:43:10.000 evaporator with dry expansion where the 00:43:12.51900:43:12.529 refrigerant flows through the tube and 00:43:13.93000:43:13.940 the liquid that means water or milk 00:43:16.99000:43:17.000 flows through the shell in direct 00:43:22.39000:43:22.400 expansion type shell and tube 00:43:23.62000:43:23.630 evaporators represent flows through the 00:43:25.48000:43:25.490 tubes while water or other liquids flows 00:43:27.22000:43:27.230 through the shell 00:43:28.90900:43:28.919 analysis of fluid flow and heat 00:43:30.80900:43:30.819 exchanger on chelsa it is very complex 00:43:32.60900:43:32.619 due to the presence of large number of 00:43:34.46900:43:34.479 tubes baffles etcetera okay so this is a 00:43:37.55900:43:37.569 complicated again design point because 00:43:39.68900:43:39.699 you can see that there are large number 00:43:40.76900:43:40.779 of tubes again large number of baffles 00:43:42.77900:43:42.789 etc okay so the prediction of heat 00:43:45.05900:43:45.069 transfer and pressure drop on the shell 00:43:47.72900:43:47.739 side is complicated because of the flow 00:43:49.89000:43:49.900 geometry okay 00:43:53.12000:43:53.130 several empirical correlations based on 00:43:55.55900:43:55.569 experimental observations have been 00:43:56.93900:43:56.949 developed over the years for predicting 00:43:59.48900:43:59.499 the pressure drop and heat transfer 00:44:01.14000:44:01.150 coefficients on the shell side for 00:44:04.76900:44:04.779 example you have a simple correlation 00:44:06.15000:44:06.160 called as emergence correlation where 00:44:08.88000:44:08.890 the nusselt number is given by HT by KF 00:44:11.51900:44:11.529 where K is the thermal conductivity of 00:44:13.19900:44:13.209 the fluid being cooled that is equal to 00:44:15.53900:44:15.549 C into re to the power of 0.6 parental 00:44:18.47900:44:18.489 number to the power of point 3 00:44:19.62000:44:19.630 multiplied by mu by mu W of 0.14 okay 00:44:23.99900:44:24.009 this looks almost similar to your C 00:44:25.97900:44:25.989 dirted correlation for flow through 00:44:28.85900:44:28.869 tubes right the viscosity effect is 00:44:31.79900:44:31.809 there Reynolds number is there in 00:44:33.02900:44:33.039 parental amperage there of course the 00:44:34.14000:44:34.150 experiments are different here we use 00:44:36.80900:44:36.819 Reynolds number to the power of 0.6 00:44:38.39900:44:38.409 whereas in case of flow through tubes we 00:44:40.70900:44:40.719 use Reynolds number to the power of 0.8 00:44:42.35900:44:42.369 as long as the flow is turbulent okay 00:44:45.44900:44:45.459 and in this expression the constant C 00:44:47.84900:44:47.859 depends on the geometry okay that means 00:44:49.82900:44:49.839 how the baffles are path pay place and 00:44:53.12000:44:53.130 how many passes are there and all that 00:44:55.58900:44:55.599 depending upon the specific geometry of 00:44:57.39000:44:57.400 the shell in tube type of evaporator the 00:45:00.02900:45:00.039 constant of C has to be obtained and 00:45:02.27900:45:02.289 used okay and the Reynolds number re D 00:45:05.48900:45:05.499 is defined as re D is Z D by mu where D 00:45:09.80900:45:09.819 is the internal diameter of the shell 00:45:11.72900:45:11.739 and G is the mass velocity which is 00:45:14.13000:45:14.140 equal to the mass flow rate divided by 00:45:16.89000:45:16.900 the characteristic flow area okay so the 00:45:19.64900:45:19.659 it has units of kg per meter square per 00:45:22.07900:45:22.089 second so find the Reynolds number from 00:45:25.94900:45:25.959 the flow rate and from the area of the 00:45:28.90900:45:28.919 or from the configuration the shell n 00:45:31.22900:45:31.239 cube evaporated then from the reynolds 00:45:34.34900:45:34.359 number and prandtl number find the 00:45:35.81900:45:35.829 nusselt number 00:45:39.29000:45:39.300 now let us look at boiling heat transfer 00:45:41.00000:45:41.010 coefficient that means heat transfer on 00:45:42.32000:45:42.330 the refrigerant side in evaporators 00:45:45.98000:45:45.990 boiling of refrigerant may take place 00:45:47.72000:45:47.730 outside tubes or inside tubes when 00:45:51.38000:45:51.390 boiling takes place outside the tubes it 00:45:53.33000:45:53.340 is called as pool boiling you might have 00:45:55.19000:45:55.200 studied this in your heat transfer basic 00:45:57.02000:45:57.030 heat transfer course pool boiling when 00:46:00.71000:46:00.720 boiling takes place inside tubes it is 00:46:02.66000:46:02.670 called as flow boiling the heat transfer 00:46:06.38000:46:06.390 coefficients in pool boiling are 00:46:07.73000:46:07.740 entirely different from that of slow 00:46:09.32000:46:09.330 boiling okay sometimes flow boiling is 00:46:13.43000:46:13.440 treated as a combination of pool boiling 00:46:15.68000:46:15.690 and force convection and correlations 00:46:17.81000:46:17.820 are formulated based on this model let 00:46:22.94000:46:22.950 us look at some of the pool boiling 00:46:24.23000:46:24.240 correlations in pool boiling the tube or 00:46:27.23000:46:27.240 the heat transfer surface is immersed in 00:46:29.35900:46:29.369 a pool of liquid which is at its 00:46:31.16000:46:31.170 saturation temperature is what is known 00:46:33.83000:46:33.840 as saturated pool boiling the heat 00:46:35.84000:46:35.850 transfer coefficient depends upon the 00:46:37.34000:46:37.350 temperature difference between the heat 00:46:38.75000:46:38.760 transfer surface and the boiling fluid 00:46:40.37000:46:40.380 there a pool boiling curve shows various 00:46:43.79000:46:43.800 stages of boiling I am sure that this 00:46:46.16000:46:46.170 you must have studied in your basic heat 00:46:47.75000:46:47.760 transfer course if you plot the pool 00:46:50.57000:46:50.580 boiling curve that means the heat flux 00:46:52.45000:46:52.460 okay mat per meter square versus the 00:46:56.18000:46:56.190 superheat okay that means the 00:46:57.44000:46:57.450 temperature difference between the 00:46:58.46000:46:58.470 surface TS is the surface TF is a fluid 00:47:01.16000:47:01.170 and if you gradually increase the degree 00:47:03.68000:47:03.690 of superheat okay you get a curve this 00:47:07.76000:47:07.770 kind of a curve right and you have 00:47:10.49000:47:10.500 different reasons here for example the 00:47:12.10900:47:12.119 freeze and one is known as a natural 00:47:15.71000:47:15.720 convection region and this reason is 00:47:20.35900:47:20.369 what is known as nucleate pool boiling 00:47:22.28000:47:22.290 region and this is your transient 00:47:24.49000:47:24.500 boiling region then this you have film 00:47:29.33000:47:29.340 boiling okay then here the radiation 00:47:33.84900:47:33.859 comes into picture okay so normally all 00:47:37.84900:47:37.859 the evaporators or boilers are designed 00:47:41.15000:47:41.160 to operate in this region okay this is 00:47:43.67000:47:43.680 what is known as your nucleate pool 00:47:47.20000:47:47.210 boiling region 00:47:51.39000:47:51.400 okay in this reason you can see that the 00:47:55.27000:47:55.280 heat heats lugs increases quite steeply 00:47:58.21000:47:58.220 with the degree of superheat Delta TS 00:48:00.94000:48:00.950 that means you get very high heat 00:48:02.65000:48:02.660 transfer coefficient in these nucleate 00:48:05.20000:48:05.210 pool boiling region okay and you can 00:48:07.30000:48:07.310 also have normally you have what is 00:48:09.76000:48:09.770 known as a critical heat flux at which 00:48:11.46000:48:11.470 the heat flux becomes maximum at a 00:48:14.29000:48:14.300 particular superheat okay so if you 00:48:16.75000:48:16.760 increase the temperature difference 00:48:18.07000:48:18.080 beyond this then the heat flux has to 00:48:23.11000:48:23.120 fall okay and this point is also known 00:48:25.30000:48:25.310 as burnout point which you must have 00:48:26.89000:48:26.900 studied in the design of boiling okay so 00:48:29.47000:48:29.480 as I said the refrigerant evaporators 00:48:32.26000:48:32.270 are designed to operate in the nucleate 00:48:34.27000:48:34.280 pool boiling regions so the correlations 00:48:36.94000:48:36.950 are available for this particular region 00:48:43.92000:48:43.930 so as I said these are already explained 00:48:47.52000:48:47.530 so now let us look at slow boiling 00:48:49.81000:48:49.820 boiling inside tubes is called as flow 00:48:51.79000:48:51.800 boiling flow boiling consists of 00:48:54.31000:48:54.320 nucleate boiling as well as convective 00:48:56.02000:48:56.030 heat transfer that means here the 00:48:58.24000:48:58.250 contribution to heat transfer comes from 00:49:00.13000:49:00.140 nucleate boiling as well as due to force 00:49:02.47000:49:02.480 convection heat transfer as the liquid 00:49:05.23000:49:05.240 evaporates more vapor is formed which 00:49:07.51000:49:07.520 increases the average velocity and the 00:49:09.22000:49:09.230 convective heat transfer rate the flow 00:49:12.10000:49:12.110 pattern changes continuously as boiling 00:49:14.56000:49:14.570 takes place along the tube for example 00:49:17.65000:49:17.660 in horizontal tube the flow can be 00:49:19.15000:49:19.160 stratified flow wavy flow slug flow 00:49:21.28000:49:21.290 annular flow mist flow okay or depending 00:49:24.49000:49:24.500 upon the velocity of the vapour now let 00:49:28.45000:49:28.460 us look at some correlations 00:49:29.71000:49:29.720 correlations for nucleate pool boiling 00:49:31.27000:49:31.280 in general x-metal there studies show 00:49:33.88000:49:33.890 that the nucleate pool boiling heat 00:49:36.79000:49:36.800 transfer coefficient h NB is equal to 00:49:39.58000:49:39.590 some C into TS minus TF ^ 2 2 3 okay 00:49:43.78000:49:43.790 where TS is the surface temperature TF 00:49:46.45000:49:46.460 is the fluid temperature and C is the 00:49:48.79000:49:48.800 constant the value of which depends upon 00:49:51.43000:49:51.440 the surface fluid combination and the 00:49:54.10000:49:54.110 exponent even though I have shown the 00:49:55.60000:49:55.610 exponent here as the varying between 2 00:49:57.64000:49:57.650 to 3 it can go up to 25 with treated 00:50:01.00000:50:01.010 surfaces okay that means with enhancer 00:50:03.07000:50:03.080 surfaces you can 00:50:04.30000:50:04.310 very very high heat transfer coefficient 00:50:06.07000:50:06.080 that means the exponent can go as high 00:50:09.13000:50:09.140 as 25 few empirical correlations are 00:50:12.13000:50:12.140 available for nucleate boiling okay this 00:50:16.75000:50:16.760 is one of the popular correlation what 00:50:18.49000:50:18.500 is known as erosion of correlation so 00:50:20.62000:50:20.630 here shears is a specific heat of liquid 00:50:23.08000:50:23.090 and delta T X is the temperature 00:50:25.12000:50:25.130 difference between the surface and the 00:50:26.35000:50:26.360 fluid hfz is the latent heat of 00:50:28.21000:50:28.220 vaporization CSF is a constant which 00:50:31.36000:50:31.370 depends on surface fluid combination 00:50:33.07000:50:33.080 which is point zero one three for 00:50:34.48000:50:34.490 halocarbons boiling on copper surface 00:50:37.06000:50:37.070 okay and Q by a here is heat flux mu F 00:50:40.99000:50:41.000 is the viscosity of the liquid hfz the 00:50:43.36000:50:43.370 latent heat of vaporization okay 00:50:45.69000:50:45.700 Sigma is the surface tension z is the 00:50:49.09000:50:49.100 acceleration due to gravity Rho F and 00:50:50.92000:50:50.930 Rosie are saturated liquid and vapor 00:50:52.45000:50:52.460 densities PRF is a liquid prantle number 00:50:56.92000:50:56.930 and the exponent is is one for water and 00:51:00.22000:51:00.230 1.74 halocarbons and all the properties 00:51:02.95000:51:02.960 have to be calculated at saturation 00:51:04.48000:51:04.490 temperature at local pressure okay so 00:51:06.67000:51:06.680 this is one of the oldest and very 00:51:08.89000:51:08.900 popular correlations for nucleate pool 00:51:10.99000:51:11.000 boiling now let us look at force 00:51:14.05000:51:14.060 convection boiling inside tubes 00:51:15.67000:51:15.680 roshun OV and griffith suggested that 00:51:18.10000:51:18.110 flow boiling in tubes be analyzed as a 00:51:20.86000:51:20.870 combination of pool boiling and force 00:51:22.69000:51:22.700 convection that means they have obtained 00:51:24.73000:51:24.740 heat total heat flux Q total is equal to 00:51:27.52000:51:27.530 Q and B plus Q SC where Q and B is the 00:51:30.79000:51:30.800 heat flux contribution because of 00:51:32.23000:51:32.240 nucleate boiling and Q subscript SC is 00:51:35.32000:51:35.330 the heat flux contribution because a 00:51:36.67000:51:36.680 force convection heat flux due to 00:51:39.04000:51:39.050 nucleate pool boiling is calculated by 00:51:41.35000:51:41.360 using nucleate pool boiling correlations 00:51:42.88000:51:42.890 for example rotational correlation and 00:51:44.92000:51:44.930 heat flux due to force convection can be 00:51:47.98000:51:47.990 calculated by using standard force 00:51:49.75000:51:49.760 convection correlations such as liters 00:51:51.79000:51:51.800 Boelter correlation okay so this is one 00:51:53.74000:51:53.750 of the simpler way of handling flow 00:51:55.96000:51:55.970 boiling okay but this is not very 00:51:59.53000:51:59.540 accurate okay gives reasonably good 00:52:01.36000:52:01.370 result but not extremely accurate we 00:52:03.64000:52:03.650 also have what is known as a bopi RIF 00:52:05.62000:52:05.630 correlation which is normally used in 00:52:07.39000:52:07.400 refrigerants this correlation gives 00:52:09.61000:52:09.620 average heat transfer coefficients and 00:52:11.29000:52:11.300 is valid for inlet quality action let 00:52:14.11000:52:14.120 varying between 0.12 0.16 that means at 00:52:17.44000:52:17.450 the exit of 00:52:18.07000:52:18.080 expansion valve and at the inlet to the 00:52:19.75000:52:19.760 evaporator the refrigerant quality 00:52:21.61000:52:21.620 should be between 0.1 to 0.16 and the 00:52:24.55000:52:24.560 correlation is given here you can see 00:52:26.11000:52:26.120 that here the nestle it is given for 00:52:27.88000:52:27.890 incomplete evaporation and it also given 00:52:29.89000:52:29.900 for complete evaporation the constant is 00:52:33.70000:52:33.710 different constant is point zero three 00:52:35.53000:52:35.540 zero is nine for incomplete evaporation 00:52:37.33000:52:37.340 and it is point zero zero eight two for 00:52:39.67000:52:39.680 complete evaporation and here re F is 00:52:42.46000:52:42.470 the Reynolds number based on the 00:52:44.35000:52:44.360 saturated liquid and KF is a constant 00:52:46.99000:52:47.000 and KF is called as a load factor and it 00:52:49.66000:52:49.670 is equal to Delta X into H F G by L 00:52:52.08000:52:52.090 where H of G the latent heat of 00:52:54.16000:52:54.170 vaporization Delta X is the change in 00:52:56.23000:52:56.240 the quality and L is the length of the 00:52:58.18000:52:58.190 tube I am sorry KF is not a constant it 00:53:00.01000:53:00.020 depends upon the particular situation 00:53:03.82000:53:03.830 and also the fluid being evaporated 00:53:07.38000:53:07.390 there is another correlation called 00:53:09.46000:53:09.470 charred oak and Brune man's correlation 00:53:11.32000:53:11.330 again you can see the correlation here h 00:53:13.75000:53:13.760 TP is the flow of flow boiling 00:53:15.19000:53:15.200 correlation h le the correlation a 00:53:18.81000:53:18.820 single-phase heat transfer coefficient 00:53:20.77000:53:20.780 of saturated referal refrigerant liquid 00:53:22.96000:53:22.970 them okay and Bo is the boiling number 00:53:25.54000:53:25.550 that is given by Q by a divided by H F Z 00:53:27.91000:53:27.920 into m dot by a where Q by a the heat 00:53:30.67000:53:30.680 flux HF G's as I said your latent heat 00:53:34.84000:53:34.850 of vaporization and m dot by a is the 00:53:36.94000:53:36.950 mass flux okay and here X T T is what is 00:53:40.84000:53:40.850 known as Lockhart and Martinelli 00:53:41.74000:53:41.750 parameter and that is defined here when 00:53:44.47000:53:44.480 xt t the small X stands for the quality 00:53:47.95000:53:47.960 Rosie and Rho F for the saturated vapor 00:53:50.35000:53:50.360 and liquid densities mu f n mu Z are the 00:53:53.02000:53:53.030 saturated liquid and vapor viscosities 00:53:56.02000:53:56.030 okay so this is one of the correlations 00:53:58.27000:53:58.280 used for flow boiling of refrigerants 00:54:01.36000:54:01.370 okay let us look at other considerations 00:54:05.80000:54:05.810 heat transfer correlations will be 00:54:07.66000:54:07.670 different for vertical tubes presence of 00:54:10.63000:54:10.640 lubricating oil affects the heat 00:54:12.07000:54:12.080 transfer coefficient and pressure 00:54:13.18000:54:13.190 pressure drop if the oil concentration 00:54:15.25000:54:15.260 is high heat transfer enhancement on 00:54:18.01000:54:18.020 refrigerant side is possible by 00:54:19.54000:54:19.550 employing force feed recirculation 00:54:21.58000:54:21.590 integral fins turbulence promoters 00:54:23.59000:54:23.600 treated surfaces etc however enhancement 00:54:27.79000:54:27.800 techniques also increase pressure drop 00:54:29.41000:54:29.420 that means we have to optimize 00:54:31.74900:54:31.759 enhancement techniques so that you get 00:54:33.42900:54:33.439 high heat transfer coefficient at the 00:54:34.74900:54:34.759 same time the pressure drop is not very 00:54:36.57900:54:36.589 high okay so these are the 00:54:38.18900:54:38.199 considerations now let us quickly look 00:54:40.44900:54:40.459 at what is known as a Wilson's plot this 00:54:43.71900:54:43.729 is a technique to determine the 00:54:45.18900:54:45.199 individual heat transfer coefficients 00:54:46.89900:54:46.909 from the exponential data on heat 00:54:48.48900:54:48.499 exchangers for example condensers and 00:54:50.22900:54:50.239 evaporators okay so from the X mental 00:54:52.38900:54:52.399 data on these heat exchangers we can 00:54:55.17900:54:55.189 find individual heat transfer 00:54:56.70900:54:56.719 coefficients for example take a water 00:54:59.25900:54:59.269 cooled condenser a number of tests can 00:55:02.13900:55:02.149 be conducted by varying the flow rate of 00:55:03.78900:55:03.799 water and measuring the inlet and outlet 00:55:05.58900:55:05.599 water temperatures from energy balance 00:55:08.94900:55:08.959 we have this equation QC is equal to M 00:55:11.28900:55:11.299 MW CPW into delta T for the water side 00:55:14.33900:55:14.349 which can be measured and this is equal 00:55:16.95900:55:16.969 to u naught a naught into L MTD since L 00:55:18.99900:55:19.009 MTD can be obtained because we are 00:55:20.40900:55:20.419 measuring the temperatures a naught is 00:55:22.41900:55:22.429 known so from this expression you can 00:55:23.88900:55:23.899 find out the overall heat transfer 00:55:25.14900:55:25.159 coefficient from the experimental 00:55:26.22900:55:26.239 measurements with negligible scale 00:55:29.62000:55:29.630 formation the expression for u naught is 00:55:31.35900:55:31.369 given here 00:55:32.07900:55:32.089 okay this we have discussed in the last 00:55:33.72900:55:33.739 class this is our expression for u 00:55:35.16900:55:35.179 naught and if the water temperature does 00:55:37.20900:55:37.219 not vary significantly then property 00:55:39.15900:55:39.169 variation will be negligible and if the 00:55:41.25900:55:41.269 flow is turbulent then the internal heat 00:55:43.41900:55:43.429 transfer coefficient that is on the 00:55:44.49900:55:44.509 water side is the proportional to V to 00:55:47.13900:55:47.149 the power of 0.8 that means H I can be 00:55:49.23900:55:49.249 written as some C into V to the power of 00:55:51.37000:55:51.380 0.8 okay as long as the property 00:55:54.03900:55:54.049 variation is not considerable and heat 00:55:57.12900:55:57.139 transfer coefficient on refrigerant side 00:55:58.80900:55:58.819 and wall resistance remain almost 00:56:00.45900:56:00.469 constant because we are not varying the 00:56:02.16900:56:02.179 refrigerant side flow rate or the 00:56:03.90900:56:03.919 operating conditions then a plot of 1 by 00:56:08.13900:56:08.149 u naught versus 1 by V to the power of 00:56:09.81900:56:09.829 point 8 will be a straight line okay so 00:56:13.92900:56:13.939 if you plot 1 by u naught okay versus 1 00:56:18.27900:56:18.289 by V to the power of point 8 then you 00:56:20.88900:56:20.899 get a straight line like this and the 00:56:22.65900:56:22.669 intercept gives you the resistance on 00:56:25.87000:56:25.880 the refrigerant side plus wall 00:56:28.12000:56:28.130 resistance because that remains constant 00:56:29.64900:56:29.659 whereas the resistance on the water side 00:56:32.40900:56:32.419 will be varying because the velocity of 00:56:34.44900:56:34.459 the water is varying okay from the 00:56:38.97900:56:38.989 intercept we can calculate the 00:56:40.29900:56:40.309 condensing heat transfer coefficient as 00:56:41.73900:56:41.749 the resistance of wall can be easily for 00:56:44.10000:56:44.110 the exponent will be different for oil 00:56:46.39000:56:46.400 cooled condensers that means for 00:56:47.83000:56:47.840 air-cooled condenser it can be done with 00:56:49.15000:56:49.160 a door water or any other fluid you have 00:56:51.16000:56:51.170 to use 0.65 in + of point 8 following 00:56:54.67000:56:54.680 resistance can be included if its values 00:56:56.35000:56:56.360 is known ok so this is the very useful 00:56:58.57000:56:58.580 concept and it is normally used for 00:57:00.31000:57:00.320 obtaining the condensing heat transfer 00:57:01.99000:57:02.000 coefficients etcetera okay so at this 00:57:04.00000:57:04.010 point I stop this lecture okay so we 00:57:07.06000:57:07.070 have completed the discussion on 00:57:08.98000:57:08.990 evaporators so I will continue this in 00:57:11.02000:57:11.030 the next lecture thank you
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